This invention relates to a catheter for the simultaneous or near-simultaneous recording of monophasic action potentials (MAPs) and ablating arrhythmia-causing tissue by coupling radiofrequency energy to tissue surrounding the catheter tip, and more particularly to a method and apparatus for simultaneously recording MAPs and ablating by contacting heart tissue with a small dual MAP/ablating electrode under positive pressure.
The electrical charge of the outer membrane of an individual heart muscle cell is known as the "membrane potential". During each heart beat, the membrane potential discharges (depolarizes) and then slowly recharges (repolarizes). The waveform of this periodic depolarization and repolarization is called the "transmembrane action potential." Mechanistically, the action potential is produced by a well-organized array of ionic currents across the cell membrane.
At the turn of the century, it had already been recognized that a potential similar in shape to the later-discovered transmembrane potential could be recorded if one brought into contact a first electrode with an injured spot of the heart and a second reference electrode with an intact spot. Those signals became known as "injury potentials" or "monophasic action potentials" (MAPs) because of the waveform shape.
The further development of the science of MAPs may be found in U.S. Pat. No. 4,955,382, the disclosure of which is hereby incorporated by reference. It has been recognized that local heart muscle injury is not a prerequisite for the generation of MAPs, and that application of slight pressure with the tip against the inner wall of the heart will result in the generation of monophasic action potential signals. These signals can be recorded reliably (i.e., without distortion) by using direct current (DC) coupled amplification.
One problem to overcome in MAP recordation is the slow DC drift caused by electrode polarization in conventional electrical material used in the recording of intracardiac electrical signals, such as silver or platinum. These materials are polarizable and cause offset and drift-which is not a problem in conventional intracardiac recordings, because those signals are AC coupled, which eliminates offset and drift. The MAPs, however, are to be recorded in DC fashion, and therefore are susceptible to electrode polarization. The use of a silver-silver chloride electrode material yields surprisingly good results in terms of both long-term stability of the signal and extremely low noise levels.
Another important discovery has been that the tip electrode of the catheter should be held against the inner surface of the heart with slight and relatively constant pressure. In order to accomplish this in a vigorously beating heart, a spring-steel stylet is inserted into a lumen of the catheter to act as an elastic spring, keeping the tip electrode in stable contact pressure with the endocardium throughout the cardiac cycle. This leads to major improvements in signal stability.
Another design objective for a good MAP catheter is to ensure a relatively perpendicular position of the electrode tip with the endocardial wall. Again, the spring electrode is useful in this respect. Conventional catheters are usually brought into contact with the heart wall in a substantially tangential manner. Such conventional catheters are designed simply to record intercardiac electrograms, not MAPs. For the monophasic action potential catheter, direct contact between the tip electrode and the endocardium is made. This also keeps the reference electrode, which is located along the catheter shaft, away from the heart muscle.
To facilitate the maneuverability of the catheter during a procedure in the human heart, the distal end of the catheter should be relatively flexible during the time of insertion, and the spring-loading feature preferably comes into action only after a stable position of the catheter tip has been obtained. Thus, in a preferred embodiment the catheter is constructed in such a way that the spring wire situated in the lumen of the catheter is retractable. During catheter insertion, the spring wire or stylet is withdrawn from its distal position by approximately 5 cm, making the tip relatively soft. Once the catheter is positioned, the spring wire is again advanced all the way into the catheter in order to stiffen it and to give it the elastic properties that are important for the described properties. Improved stylets and wires that position the tip electrode perpendicularly to the heart surface with the proper amount of constant pressure are described in U.S. Pat. No. 4,955,382.
Thus, a main feature of a good catheter is the ability to bring into close and steady contact with the inner surface of the myocardial wall a nonpolarizable electrode which both produces and records MAPs. To achieve this property, the electrodes are formed from nonpolarizable material such as silver-silver chloride, and the tip electrode should be maintained at a relatively constant pressure against the myocardial wall, preferably with some type of spring loading.
The catheter of the present invention preferably contains a spring-steel guide wire which provides a high degree of elasticity or resilience, allowing the catheter tip to follow the myocardial wall throughout the heartbeat without losing its contacting force and without being dislodged. The inner surface of the heart is lined with crevices and ridges (called the trabeculae carneae) that are helpful in keeping the spring-loaded catheter tip in its desired location. The contact pressure exerted by the tip electrode against the endocardial wall is strong enough to produce the amount of local myocardial depolarization required to produce the MAP. The contact pressure is, on the other hand, soft and gentle enough to avoid damaging the endocardium or the myocardium or cause other complications. In particular, no cardiac arrhythmias are observed during the application of the catheter. Usually a single extra beat is observed during the initial contact of the catheter tip against the wall.
The tip electrode is responsible for the generation and the recording of the MAP itself. A reference electrode, or "indifferent" electrode, required to close the electrical circuit, is located approximately 3 to 5 mm from the tip electrode in the catheter shaft and is embedded in the wall so that it is flush with or slightly recessed in the catheter shaft. In this position, the electrode makes contact only with the surrounding blood and not with the heart wall itself.
This reference electrode is brought into close proximity with the tip electrode, since the heart as a whole is a forceful electrical potential generator and these potentials are everywhere in the cardiac cavities. If the reference electrode were in a remote location, then the amplifier circuit would pick up the QRS complex.
Another feature of this invention relates to thermal destruction, or ablation. Ablation of abnormal myocardial tissue (such as arrhythmia-causing tissue) is a therapeutic procedure used with increasing frequency for treatment of cardiac arrhythmias such as, for example, ventricular tachycardia. The medical technique of ablation is discussed in G. Fontaine et al., Ablation in Cardiac Arrhythmias (New York: Futura Publishing Co., 1987), and D Newman et al., "Catheter Ablation of Cardiac Arrhythmias", in Current Problems in Cardiology, Year Book Medical Publishers, 1989.
Catheter ablation of ventricular tachycardia was first described in 1983 as a method for destroying arrhythmia-causing tissue. Typically, a pacing catheter is introduced into the left ventricle of the heart, and manipulated until the site of earliest activation during ventricular tachycardia is found, indicating the location of the problem tissue. Electrical energy, often high voltage DC pulses are then applied between a catheter-mounted electrode and an external chest wall electrode. In this way, arrhythmia-causing cardiac tissue is destroyed.
More recently, less drastic methods than high voltage pulses have been developed, which are painful (requiring general anaesthesia), and dangerous due to arcing and explosive gas formation at the catheter tip. The use of electromagnetic energy, more particularly radiofrequency (RF) or microwave energy, is currently in popular use. RF and microwave energy, unless otherwise noted, refers to energy in the electromagnetic spectrum from about 10 kHz to 100 GHz. RF ablation, usually in the range of 300-1200 kHz, is a safer alternative to high voltage DC pulsing in which RF energy is applied to the endocardium via an catheter electrode. Tissue destruction, or ablative injury, is effected by heating generated by the RF electric field. RF ablation results in a more controllable lesion size, with no gas or shock wave formation. Ablation may also be effected with energy having microwave frequencies, from about 700 MHz to 100 GHz.
Currently, no reliable on-line method exists to quickly and accurately determine whether radiofrequency (RF) energy application has resulted in lesions of a size sufficient to destroy the injured myocardial tissue. That is primarily because previously, no provision has been made for accurately measuring the electrophysiological activity of the heart in the immediate vicinity of heart tissue which is being ablated by an ablating catheter. Moreover, if it is desired to pace the heart at the same time as measuring MAPs in the heart, two entrance sites to the patient must be created and two catheters must be utilized, which is highly undesirable.
Because of the complexity of electrical cardiac activity, when a pacing or ablating electrode is inserted into the heart, and it is desired to measure the resulting monophasic action potentials of the heart, it would be of extreme usefulness to be able to measure such potentials in the vicinity of the ablation, rather than at a more remote location.
Important applications of the present invention are in the areas of studying and treating myocardial ischemia and cardiac arrhythmias. In particular, the present invention permits (1) precisely locating areas of myocardial ischemia by studying localized MAPs and directly treating them; and (2) diagnosing the nature and locality of arrhythmias originating from after-depolarizations and treating those arrhythmias. These after-depolarizations have heretofore been detected only in isolated animal tissue preparations where microelectrodes can be applied. The MAP/ablation catheter is a tool that can allow the clinical investigator to detect and remedy such abnormal potentials in the human heart and thereby significantly broaden the ability to diagnose this group of arrhythmias.